In a working vehicle such as a bulldozer, lubricating oil is supplied to the transmission device and the steering device. The lubricating oil supplied to the transmission device and the steering device is recovered in a transmission case for housing the transmission device and a steering case for housing the steering device, respectively, and is reused (see Japanese Laid-Open Patent Publication No. 2004-211665).
The structure of a conventional hydraulic system for supplying lubricating oil to the transmission device and the steering device is shown in FIG. 5. In the conventional hydraulic system, oil stored in the steering case 81 is supplied to a steering lubricating circuit 83 by means of a low-pressure pump 82. The oil is then supplied as the lubricating oil to the steering device 84 via the steering lubricating circuit 83. The oil used as the lubricating oil drips down from the steering device 84, and is recovered in the steering case 81.
The oil stored in the steering case 81 is supplied to a high-pressure circuit 86 by means of a high-pressure pump 85. The oil is then supplied to the clutch 87 of the transmission device via the high-pressure circuit 86, whereby the clutch 87 is driven. A low-pressure circuit 89 is connected to the high-pressure circuit 86 via a relief valve 88. The oil stored in the steering case 81 is also supplied to the low-pressure circuit 89 by means of the high-pressure pump 85. The oil supplied to the low-pressure circuit 89 passes through a torque converter 90, and is then supplied as lubricating oil to a transmission device 91. The oil used as the lubricating oil drips down from the transmission device 91, and is recovered in a transmission case 92.
The oil stored in the transmission case 92 is returned to the steering case 81 by means of a scavenging pump 93 via a scavenging circuit 94, the steering lubricating circuit 83, and the steering device 84.